Thanks to the study of R.H. Charles, this hypothesis, with clear-cut evidence, can be postulated: the 'Vision of Isaiah' started as a Jewish Greek text which got interpolated later by Docetist Christians.
R.H. Charles made a study on the "vision" part of the 'Ascension of Isaiah' from the available sources. Next, his graphic shows how the different versions of the book (available or not) fit together:
Here is the text in question (with my notes):
1. AND the vision which Isaiah saw, he told to Hezekiah and Josab his son and Micaiah and the rest of the prophets, (and) said:
2. At this moment, when I prophesied according to the (words) heard which ye heard, I saw a glorious angel not like unto the glory of the angels which I used always to see, but possessing such glory ad position that I cannot describe [[[the glory of that angel]]].
3. And having seized me by my hand he raised me on high, and I said unto him: "Who art thou, and what is thy name, and whither art thou raising me on high? for strength was given me to speak with him."
4. And he said unto me: "When I have raised thee on high [though the (various) degrees] and made thee see the vision, on account of which I have been sent, then thou wilt understand who I am: but my name thou dost not know.
5. Because thou wilt return into this thy body, but whither I am raising thee on high, thou wilt see; [[[for this purpose have I been sent.]]]"
6. And I rejoiced because he spake courteously to me.
7. And he said unto me: "Hast thou rejoiced because I have spoken courteously to thee?" And he said: "And thou wilt see how a grater also that I am will speak courteously and peaceably with thee."
8. And [[His Father also who is greater]] thou wilt see; for for this purpose have I been sent from the seventh heaven in order to explain all these things unto thee."
9. And we ascended to the firmament, I and he, and there I saw Sammael and his hosts, and there was great fighting therein and the angels of Satan were envying one another.
10. And as above so on the earth also; for the likeness of that which is in the firmament is here on he earth.
11. And I said unto the angel (who was with me): "(What is this war and) what is this envying?"
12. And he said unto me: "So has it been since this world was made until now, and this war (will continue) till He, whom thou shalt see will come and destroy him."
13. And afterwards he caused me to ascend (to that which is) above the firmament: which is the (first) heaven.
14. And there I saw a throne in the midst, and on his right and on his left were angels.
15. [[[And (the angels on the left were) not like unto the angels who stood on the right,]]]but those who stood on the right had the greater glory, and they all praised with one voice, [[[and there was a throne in the midst]]], and those who were out the left gave praise after them; but their voice was not such as the voice of those on the right, nor their praise like the praise of those.
16. And I asked the angel who conducted me, and I said unto him: "To whom is this praise sent?"
17. And he said unto me: "(it is sent) to the praise of (Him who sitteth in) the seventh heaven: to Him who rests in the holy world, and to His Beloved, whence I have been sent to thee. [Thither is it sent.]"
18. And again, he made me to ascend to the second heaven. now the height of that heaven is the same as from the haven to the earth [and to the firmament].
19. And (I saw there, as) in the first heaven, angels on the right and on the left, [[[and a throne in the midst, and the praise of the angels in the second heaven; and he who sat on the throne in the second heaven was more glorious than all (the rest)]]].
20. And there was great glory in the second heaven, and the praise also was not like the praise of those who were in the first heaven.
21. And I fell on my face to worship him, but he angel who conducted me did not permit me, but said unto me: "Worship neither throne nor angel which belongs to the six heavens - for for this cause I was sent to conduct thee j- until I tell thee [[[in the seventh heaven]]].
22. For above all the heavens and their angels has thy throne been placed, and thy garments and thy crown which thou shalt see."
23. And I rejoiced with great joy, that those who love the Most High and His Beloved will afterwards ascend thither by the angel of the Holy Spirit.
24. And he raise me to the third heaven, and in like manner I saw those upon the right and upon the left, and there was a throne there in the midst; but the memorial of this world is there unheard of.
25. [[[And I said to the angel who was with me;]]] for the glory of my appearance was undergoing transformation as I ascended to each heaven in turn: "Nothing [[[of the vanity]]] of that world is here named."
26. And he answered me, and said unto me: "Nothing is named on account of its weakness, and nothing is hidden there of what is done."
27. [[[And I wished to learn how it is know, and he answered me saying: "When I have raised thee to the seventh heaven whence I was sent, to that which is above these, then thou shalt know that there is nothing hidden from the thrones and from those who dwell in the heavens and from the angels.]]] And the praise wherewith they praised and glory of him who sat on the throne was great, [[[and the glory of the angels on the right hand and on the left was beyond that of the heaven which was below them.]]]
28. And again he raised me to the fourth heaven, and the height from the third to the height from the third to the forth heaven was greater than from the earth to the firmament.
29. And there again I saw those who were on the right hand and those who were on the left, [[[and him who sat on the throne was in the midst,]]] and there also they were praising.
30. And the praise and glory of the angels on the right was greater than that of those on the left.
31. And again the glory of him who sat on the throne was greater than that of the angels on the right, and their glory was beyond that of those who were below.
32. And he raised me to the fifth heaven.
33. And again I saw [[[those upon the right hand and on the left, and him who sat on the throne possessing greater glory that those of the forth heaven.]]]
34. And the glory of those on the right hand was greater than that of those [[[on the left]]] [from the third to the fourth].
35. [[[And the glory of him who was on the throne was greater than that of the angels on the right hand.]]]
36. And their praise was more glorious than that of the fourth heaven.
37. [[[And I praised Him, who is not named and the Only-begotten]]] who dwelleth in the heavens, whose name is not known to any flesh, who has bestowed such glory on the several heavens, [[[and who makes great the glory of the angels, and more excellent the glory of Him who sitteth on the throne.]]]
1. AND again he raised me into the air of the sixth heaven, and I saw such glory as I had not seen in the five heavens.
2. For I saw angels possessing great glory.
3. And the praise there was holy and wonderful.
4. And I said to the angel who conducted me: "What is this which I see, my Lord?"
5. And he said: "I am not thy lord, but thy fellow servant."
6. [[[And again I asked him, and I said unto him: "Why are there not angelic fellow servants (on the left)?"]]]
7. And he said: "From the sixth heaven there are no longer angels on the left, nor a throne set in the midst, but (they are directed) by the power of the seventh heaven, where dwelleth He that is not named [[[and the Elect One, whose name has not been made known, and none of the heavens can learn His name.]]]
8. For it is He alone to whose voice all the heavens and thrones give answer. I have therefore [[[been empowered and]]] sent to raise thee here that thou mayest see this glory.
9. And that thou mayest see the Lord of all those heavens and these thrones.
My note: this Lord appears to be God himself.
10. [[[Undergoing (successive) transformation until He resembles your form and likeness.]]]
11. I indeed say unto thee, Isaiah; No man about to return into a body of that world has [[[ascended or]]] seen what thou seest or perceived what thou hast perceived and what thou wilt see.
12. For it has been permitted to thee in the lot of the Lord to come hither. [And from thence comes the power of the sixth heaven and of the air]."
13. And I magnified my Lord with praise, in that through His lot I should come hither.
14. And he said: "[[[Hear, furthermore, therefore, this also from thy fellow servant:]]] when from the body by the will of God thou hast ascended hither, then thou wilt receive the garment [[[which thou seest, and likewise other numbered garments laid up (there) thou wilt see]]].
15. And then thou wilt become equal to the angels of the seventh heaven.
16. And he raised me up into the sixth heaven, and there were no (angels) on the left, nor a throne in the midst, but all had one appearance and their (power of) praise was equal.
17. And (power) was given to me also, and I also praised along with them and that angel also, and our praise was like theirs. 18. And there they [[all named the primal Father]] and His Beloved, [[[the Christ,]]] and the Holy Spirit, all with one voice.
19. And (their voice) was not like the voice of the angels in the five heavens.
20. [Nor like their discourse] but the voice was different there, and there was much light there.
21. And then, when I was in the sixth heaven I thought the light which I had seen in the five heavens to be but darkness.
22. And I rejoiced and praised Him who hath bestowed such lights on those who wait for His promise.
23. And I besought the angel who conducted me that I should not henceforth return to the carnal world.
24. I say indeed unto you, [[[Hezekiah and Josab my son and Micaiah,]]] that there is much darkness here.
25. And the angel who conducted me discovered what I thought and said: "If in this light thou dost rejoice, how much more wilt thou rejoice, when in the seventh heaven thou seest the light where is the Lord and His Beloved [whence I have been sent, who is to be called "Son" in this world.
26. Not (yet) hath been manifested he shall be in the corruptible world] and the garments, and the thrones, and the crowns which are laid up for the righteous, [[[for those who trust in that Lord who will descend in your form. For the light which is there is great and wonderful.]]]
27. And as concerning thy not returning into the body thy days are not yet fulfilled for coming here."
28. And when I heard (that) I was troubled, and he said: "Do not be troubled."
1. AND he took me into the air of the seventh heaven, and moreover I heard a voice saying: "How far will he ascend that dwelleth in the flesh?" And I feared and trembled.
2. And [[[when I trembled, behold,]]] I heard from hence another voice being sent forth, and saying: "It is permitted to the holy Isaiah to ascend hither; for here is his garment."
3. And I asked the angel who was with me and said: "Who is he who forbade me and who is he who permitted me to ascend?"
4. And he said unto me: "He who forbade thee, is he who is over the praise-giving of the sixth heaven.
5. And He who permitted thee, [[[this is thy Lord God, the Lord Christ, who will be called "Jesus" in the world,]]] but His name thou canst not hear till thou hast ascended out of thy body."
6. And he raised me up into the seventh heaven, and I saw there a wonderful light and angels innumerable.
7. [[[And there I saw the holy Abel and all the righteous.
8. And there I saw Enoch and all who were with him,]]] stript of the garments of the flesh, and I saw them in their garments of the upper world, and they were [[[like angels]]], standing there in great glory.
9. [[[And there I saw Enoch and all who were with him, stript of the garments of the flesh, and I saw them in their garments of the upper world, and they were like angels, standing there in great glory.]]]
10. But they sat not on their thrones, nor were their crowns of glory on them.
11. And I asked the angel who was with me: "How is it that they have received the garments, but have not the thrones and the crowns?"
12. And he said unto me: "Crowns and thrones of glory they do not receive, till the Beloved will descent in the form in which you will see Him descent [will descent, I say] [[[into the world in the last days the Lord, who will be called Christ.]]]
13. Nevertheless they see and know whose will be thrones, and whose the crowns when He has descended and been made in your form, [[[and they will think that He is flesh and is a man.]]]
14. And the god of that world will stretch forth **his hand against the Son,** and they will crucify Him on a tree, and will slay Him not knowing who He is.
1. "and been made in your form" and this "Christian" verse seem to be an out-of-context insertion on the original text, disrupting the flow of the narration.
2. The underlined words in this verse and in verses 13 & 16 are more likely from the first (Christian) interpolation.
15. And thus His descent, [[[as you will see, will be hidden even from the heavens, so that it will not be known who He is.]]]
16. And when He hath plundered the angel of death, He will ascend on the third day [and he will remain in that world five hundred and forty-five days].
1. Another suspected insertion in the original Greek text: why wait two to three days after having plundered the angel of death?
2. The 545 days (1.5 year) are in line with the duration of Jesus' sojourn on earth according to Marcion's gospel. That probably started as a note in the margin which then got moved into the text.
17. And then many of the righteous will ascend with Him, whose spirits do not receive their garments till the [[[Lord Christ]]] ascend and they ascend with Him.
18. Then indeed they will receive their [garments and] thrones and crowns, when He has ascended into the [[[seventh heaven.]]]"
19. And I said unto him that which I had asked him in the third heaven:
20. "Show me how everything which is done in that world is here made known."
21. And whilst I was still speaking with him, behold one of the angels who stood nigh, more glorious than the glory of that angel, who had raised me up from the world.
22. Showed me a book, [but not as a book of this world] and he opened it, and the book was written, but not as a book of this world. And he gave (it) to me and I read it, and lo! the deeds of the children of Israel were written therein, and the deeds of those whom I know (not), my son Josab.
23. And I said: "In truth, there is nothing hidden in the seventh heaven, which is done in this world."
24. And I saw there many garments laid up, and many thrones and many crowns.
25. And I said to the angel: "Whose are these garments and thrones and crowns?"
26. And he said unto me: "These garments many from that world will receive, believing in the words of That One, [[[who shall be named]]] as I told thee, [[[and they will observe those things, and believe in them, and believe in His cross: for them are these laid up.]]]"
27. And I saw a certain One [[[standing, whose glory surpassed that of all]]], and His glory was great [[[and wonderful]]].
My note: that certain One is most likely God himself.
28. [[[And after I had seen Him,]]] all the righteous whom I had seen [[[and also the angels whom I had seen]]] came to Him. [[[And Adam and Abel and Seth and all the righteous first drew near]]] and worshipped Him, and they all praised Him with one voice, [[[and I myself also gave praise with them]]], and my giving of praise was as theirs.
29. And then all the angels drew nigh and worshipped and gave praise.
30. And I was (again) transformed and became like an angel.
31. And thereupon the angel who conducted me said to me: "Worship this One," and I worshipped and praised.
32. And the angel said unto me: "This is the Lord of all the praise-givings which thou hast seen."
My note: this is the Lord (= God) being praised by angels during the ascent of Isaiah.
33. And whilst he was still speaking, I saw another Glorious One who was like Him,
My note: this "Glorious one", in the image of God, appears to be the Beloved. Confirmation next.
and the righteous drew nigh and worshipped and praised, and I praised together with them. But my glory was not transformed into accordance with their form.
34. And thereupon the angels drew near and worshipped Him.
35. And I saw the Lord and the second angel, and they were standing.
My note: the "Glorious one" is implied to be an angel, the first one of the twosome, regardless if here "Lord" means God or the Glorious one. That does not seem Christian.
36. And the second whom I saw was on the left of my Lord. And I asked: "Who is this?" and he said unto me: "Worship Him, for He is the angel of the Holy Spirit, who speaketh in thee and the rest of the righteous."
My note: the Holy Spirit is an angel also. Again, not a Christian concept!
37. And I saw the great glory, the eyes of my spirit being open, and I could not thereupon see, nor yet could the angel who was with me, nor all the angels whom I had seen worshipping my Lord.
38. But I saw the righteous beholding with great power the glory of that One.
39. And my Lord drew nigh to me and the angel of the Spirit [[[and He said: "See how it is given to thee to see God, and on thy account power is given to the angel who is with thee."
40. And I saw how my Lord and the angel of the Spirit]]] worshipped, and they both together praised [[[God]]].
My note: "my Lord" here means the Beloved (and not God), because both worshipped & praised (God would not worship or praise himself). From now on, in the rest of the text, "my Lord" will be the Beloved (see 9:12, where the Beloved is predicted to descend).
41. And thereupon all the righteous [[[drew near and]]] worshipped.
42. And the angels [[[drew near and]]] worshipped and all the angels praised.
1. AND thereupon I heard the voices and the giving of praise, which I had heard in each of the six heavens, ascending and being heard there:
2. And all [[[were being sent up to that Glorious One]]] whose glory I could not behold.
3. [[[And I myself was hearing and beholding the praise (which was given) to Him.
4. And the Lord and the angel of the Spirit were beholding all and hearing all.]]]
5. And all the praises which are sent up from the six heavens are not only heard, but seen.
6. And [[[I heard]]] the angel [[[who conducted me and]]] he said: "This is the Most High of the high ones, dwelling in the holy world, and resting in His holy ones, who will be called by the Holy Spirit through the lips of the righteous [[[the Father of the Lord.]]]"
7. And I heard the voice of the Most High, [[[the Father of my Lord, saying to my Lord Christ who will be called Jesus]]]:
8. "Go forth and descent through all the heavens, and thou wilt descent to the firmament and that world: to the angel in Sheol thou wilt descend, [[[but to Haguel thou wilt not go.]]]
My note: it does not look here the Sacrifice was part of God's plan, despite what Paul said in his epistles!
9. And thou wilt become like unto the likeness of all who are in the five heavens.
10. And thou wilt be careful to become like the form of the angels of the firmament [and the angels also who are in Sheol].
11. And none of the angels of that world shall know [[[that Thou art with Me of the seven heavens and of their angels.
12. And they shall not know that Thou art with Me, till with a loud voice I have called (to) the heavens, and their angels and their lights, (even) unto the sixth heaven, in order that you mayest]]] judge [[[and destroy the princes]]] and angels [[[and gods]]] of that world, and the world that is dominated by them:
13. For they have denied Me and said: "We alone are and there is none beside us."
And afterwards from the angels of death Thou wilt ascend to Thy place. And Thou wilt not be transformed in each heaven, but in glory wilt Thou ascend and sit on My right hand.
My note: no mention of crucifixion/Sacrifice between verse 8 & 14!
15. And thereupon the princes and powers of that world will worship Thee."
16. These commands I heard the Great Glory giving to my Lord.
17. And so I saw my Lord go forth from the seventh heaven into the sixth heaven.
18. And the angel who conducted me [from this world was with me and] said unto me: "Understand, Isaiah, and see the transformation and descent of the Lord will appear."
19. And I saw, and when the angels saw Him, thereupon [[[those in the sixth heaven]]] praised and lauded Him; for He had not been transformed after the shape of the angels there, and they praised Him and I also praised with them.
20. And I saw when He descended into the fifth heaven, that in the fifth heaven He made Himself like unto the form of the angels there, and they did not praise Him (nor worship Him); for His form was like unto theirs.
21. And then He descended into the forth heaven, and made Himself like unto the form of the angels there.
22. And [[[when they saw Him,]]] they did not praise [[[or laud Him;]]] for His form was like unto their form.
23. And again I saw when He descended into the third heaven, [[[and He made Himself like unto the form of the angels in the third heaven.
24. And those who kept the gate of the (third) heaven demanded the password, and the Lord gave (it) to them in order that He should not be recognized. And when they saw Him, they did not praise or laud Him; for His form was like unto their form.
25. And again I saw when He descended]]] into the second heaven, [[[and again He gave the password there; those who kept the gate proceeded to demand and the Lord to give.
26. And I saw when He made Himself like unto the form of the angels in the second heaven, and they saw Him and they did not praise Him; for His form was like unto their form.
27. And again I saw when He descended]]] into the first heaven, [[[and there also He gave the password to those who kept the gate, and He made Himself like unto the form of the angels who were on the left of that throne,]]] and they neither praised nor lauded Him; for His form was like unto their form.
28. [[[But as for me no one asked me on account of the angel who conducted me.]]]
29. And again He descended into the firmament [[[where dwelleth the ruler of this world,]]] and He gave the password [[[to those on the left,]]] and His form was like theirs, and they did not praise Him there; [[[but they were envying one another and fighting; for here there is a power of evil and envying about trifles.]]]
My note: nothing happens here. Satan is not even mentioned.
30. And I saw when He descended [[[and made Himself like]]] unto the angels of the air, and He was like one of them.
31. And He gave no password; for one was plundering and doing violence to another.
My note: these angels of the air are too busy fighting each other to pay attention to the Beloved, who is transformed just like them. How could the Beloved/Lord be arrested in the air in these circumstances?
1. AFTER this [[[I saw, and]]] the angel [[[who spoke with me, who conducted me,]]] said unto me: "Understand, Isaiah son of Amoz; for this purpose have I been sent from God."
2. [[[And I indeed saw a woman of the family of David the prophet, named Mary, and Virgin, and she was espoused to a man named Joseph, a carpenter, and he also was of the seed and family of the righteous David of Bethlehem Judah.
3. And he came into his lot. And when she was espoused, she was found with child, and Joseph the carpenter was desirous to put her away.
4. But the angel of the Spirit appeared in this world, and after that Joseph did not put her away, but kept Mary and did not reveal this matter to any one.
5. And he did not approach Mary, but kept her as a holy virgin, though with child.
6. And he did not live with her for two months.
7. And after two months of days while Joseph was in his house, and Mary his wife, but both alone.
8. It came to pass that when they were alone that Mary straight-way looked with her eyes and saw a small babe, and she was astonished.
9. And after she had been astonished, her womb was found as formerly before she had conceived.
10. And when her husband Joseph said unto her: "What has astonished thee?" his eyes were opened and he saw the infant and praised God, because into his portion God had come.
11. And a voice came to them: "Tell this vision to no one."
12. And the story regarding the infant was noised broad in Bethlehem.
13. Some said: "The Virgin Mary hath borne a child, before she was married two months."
14. And many said: "She has not borne a child, nor has a midwife gone up (to her), nor have we heard the cries of (labour) pains." And they were all blinded respecting Him and they all knew regarding Him, though they knew not whence He was.
15. And they took Him, and went to Nazareth in Galilee.
16. And I saw, O Hezekiah and Josab my son, and I declare to the other prophets also who are standing by, that (this) hath escaped all the heavens and all the princes and all the gods of this world.
17. And I saw: In Nazareth He sucked the breast as a babe and as is customary in order that He might not be recognized.
18. And when He had grown up he worked great signs and wonders in the land of Israel and of Jerusalem.
19. And after this the adversary envied Him and roused the children of Israel against Him,]]] not knowing who He was, [[[and they delivered Him to the king, and crucified Him, and He descended to the angel (of Sheol).
20. In Jerusalem indeed I was Him being crucified on a tree:
21. And likewise after the third day rise again and remain days.
22. And the angel who conducted me said: "Understand, Isaiah": and I saw when He sent out the Twelve Apostles and ascended.]]]
1. This mini-gospel, probably inspired by Matthew's one, has Gnostic and Docetist elements.
2. But why describe the Beloved's descent from heaven (with the purpose of liberating souls of the dead) and then his ascent back to God, without describing what the Beloved did below earth in Sheol?
I don't think it was necessary: the main obstacle was to proceed through the layers of heaven without being intercepted, which the Beloved is described to have done successfully. And vanquishing the angel of death in Sheol is implied by the Beloved's returning to heaven in full glory.
3. The text was just meant to give hope, that God, through his Beloved, and despite their assumed isolation in the highest heaven because of the hostility of angels & Satan in the lower heavens, would take action in order to save dead Jews.
4. When was the original 'Vision of Isaiah' written? Logically, right after many Jews died, that is in the first Jewish war.
23. And I saw Him, and He was in the firmament, but He had not changed Himself into their form, and all the angels of the firmament [[[and the Satans]]] saw Him and they worshipped.
24. [[[And there was much sorrow there, while]]] they said: "How did our Lord descend in our midst, and we perceived not the glory [which has been upon Him], which we see has been upon Him from the sixth heaven?"
25. And He ascended into the second heaven, and He did not transform Himself, but all the angels who were on the right and on the left and the throne in the midst.
26. Both worshipped Him and praised Him and said: "How did our Lord escape us whilst descending, and we perceived not?"
27. And in like manner He ascended into the third heaven, [[[and they praised and said in like manner.]]]
28. And in the fourth heaven and in the fifth [[[also they said precisely after the same manner.
29. But there was one glory, and from it He did not change Himself.
30. And I saw when He ascended]]] into the sixth heaven, [[[and they worshipped and glorified Him]]].
31. But in all the heavens the praise increased (in volume).
32. And I saw how He ascended into the seventh heaven, and all the righteous and all the angels praised Him. And then I saw Him sit down on the right hand of that Great Glory whose glory I told you that I could not behold.
33. And also the angel of the Holy Spirit I saw sitting on the left hand.
34. And this angel said unto me: "Isaiah, son of Amoz, [[it is enough for thee;]]... for thou hast seen what no child of flesh has seen.
35. And thou wilt return into thy garment (of the flesh) until thy days are completed. Then thou wilt come hither."
36. These things Isaiah saw and told unto all that stood before him, and they praised. And he spake to Hezekiah the King [[[and said:]]] "I have spoken these things."
37. Both the end of this world;
38. And all this vision will be consummated in the last generations.
39. And Isaiah made him swear that he would not tell (it) to the people of Israel, nor give these words to any man to transcribe.
40. ...such things ye will read. and watch ye in the Holy Spirit in order they ye may receive your garments and thrones and crowns of glory which are laid up in the seventh heaven.
41. On account of these visions and prophecies Sammael Satan sawed in sunder Isaiah the son of Amoz, the prophet, by the hand of Manasseh.
42. And all these things Hezekiah delivered to Manasseh in the twenty-sixth year.
43. But Manasseh did not remember them nor place these things in his heart, but becoming the servant of Satan he was destroyed. Here endeth the vision of Isaiah the prophet with his ascension
It seems the original text was entirely Jewish and then slightly interpolated (at 9:13-14 & 16) by Docetic Christian(s). Then two interpolators (Docetic Christians themselves), separately, from their own copy, added more DIFFERENT additions/insertions, some of them overtly Christian in nature. These two resulting copies (witnessed by E & L1 and S & L2) were furthermore interpolated when new ones were made from them.
The original text, even after the first Christian-like interpolations, still did not have "Son", "first-begotten", "Jesus" or "Christ" in it (that will come later from different interpolators).
Therefore, the 'Vision of Isaiah' and descent were not originally Christian. But later, probably in the 2nd century, when Docetism was adopted among some Gnostic Christians, the original Jewish text got manipulated in order to serve the Christian cause.
According to Irenaeus' Against Heresies, 1, 5, 2: "He created also seven heavens, above which they say that he, the Demiurge [the lesser God of the Jews and creator of the universe] exists. And on this account they [heretics] term him Hebdomas, and his mother Achamoth Ogdoads, preserving the number of the first-begotten [Christ] and primary Ogdoad [the supreme God] as the Pleroma. They affirm, moreover, that these seven heavens are intelligent, and speak of them as being angels, while they refer to the Demiurge himself as being an angel bearing a likeness to God.."
That would explain why those 2nd century heretics would find a fully Jewish 'Vision of Isaiah' as a readily available basis apt to be modified by their Gnostic/Docetic interpolations.
But let's note, in the 'Vision of Isaiah', the supreme God (and the Beloved's boss) is also the God of Israel (9:22) and there is no Demiurge. That would show the original text was Jewish and not written by a 2nd century Christian Gnostic.
I know that this thinking is very much far removed from Dr. Carrier's observations in OHJ (page 41) on 'the 'Vision of Isaiah':
The 'they' who will think he is and not know who he is and kill him are only ever said to be Satan and angels. No other subject is mentioned for that pronoun, nor is any other implied. God clearly intends Jesus to do nothing more than go to the firmament, and for no other reason than to be killed by Satan and his sky demons, rise from the dead and conduct affairs there for over a year (doing what, it's not said), and then ascend to heaven. In other words, instead of conducting a ministry on earth, Jesus is commanded to go straight to the end and die, and rise from the dead, and then remain where he had for a year and a half (9.16; cf. 10.12-14; although the duration is omitted in some versions), and then ascend to the heavens. The 'tree' on which is crucified (9.14) is thus implied to be one of the 'copies' of trees that we are told are in the firmament (7.10).
Of course, even with all its (highly suspected) interpolations, the text of 'vision of Isaiah' does not say what I bolded from page 41 of Carrier's 'On the Historicity Of Jesus' (OHJ). This is just pure speculations.
Furthermore, Paul indicated Jesus' crucifixion happened in the heartland of the Jews, among Jews.